The proliferation of digital information has created a new television industry employing the concept of a "digital studio", e.g., the HDTV (High Definition Television) or SDTV (Standard Definition television) broadcast studio. A digital studio is an environment or system having numerous components where various sources of digital information can be selectively accessed, manipulated and delivered (in real time or in delay mode) to multiple clients.
Currently, a digital studio is required to produce an output data stream that meets the specifications set forth in the ATSC (Advanced Television Systems Committee) Digital Television Standard and the MPEG-2 systems level standards as set forth in ISO/IEC 13818-1 recommendation H.222.0. The digital studio is required to dynamically switch between various program sources and to produce a compliant output stream. Program sources include, but are not limited to, file servers, tape players, encoders, satellite links, networks and other sources, where these sources may contain either pre-recorded or "live" data streams. The digital studio may incorporate a switcher, e.g., a Play-To-Air Switcher, to switch or splice the various data streams into a single output stream.
However, transport streams that serve as inputs to the Play-to-Air Switcher are generally plesiochronous (not sourced from the same clock but are nominally at the same frequency with a defined precision), since these input transport streams are from a variety of sources and are likely created or recorded at different times. In other words, the timing information (byte rate and Program Clock Reference as discussed below) of each input transport stream is determined by the clocks used at the time of the recording, which may vary depending upon the local conditions that prevail at that time. For example, MPEG defines timing information in terms of time stamps or clock references, e.g., "System Clock Reference" (SCR) and "Program Clock Reference" (PCR), which indicate the time of a specific action such as the arrival of a byte or the presentation of a presentation unit, e.g., a decoded picture or a decoded audio frame.
Currently, the output stream is required to contain a PCR that has a frequency tolerance of 30 ppm and a rate of change specification of less than or equal to 75.times.10.sup.-3 Hz/sec. This requirement permits the PCR to change one cycle in frequency over 131/3 seconds. The MPEG specification further notes that "sources of coded data should follow a tighter tolerance in order to facilitate compliant operation of consumer recorders and playback equipment." (ISO/IEC 13818-1 .sctn.2.4.2.1)
The diversity in the timing information of the various input transport streams creates a difficult dilemma for the digital studio. The digital studio may adjust its clock to accommodate the timing of a particular input transport stream. For example, if a particular input transport stream is clocked from the storage device (e.g., a server) using a clock that is lower in frequency than the clock used to record the stream, the digital studio can compensate for the lower output rate by maintaining a PCR at a comparable lower rate in its output transport stream. Unfortunately, switching from one input transport stream to another presents the problem of matching the clock rates without violating the PCR slew rate specification. Namely, changing the clock rate too rapidly may cause the clock located on the receiver (possibly including the transmitter for the digital studio) to lose frequency lock, thereby resulting in the loss of pictures or sudden shifting or "jerking" in the presentation of video data and/or similar detrimental effects in the presentation of audio data.
Furthermore, ATSC Standard (Doc. A/53, Sep. 16, 1995) specifically states that the clock frequencies between the source coding domain and the channel coding domain operate asynchronously and are not required to be frequency-locked to each other. The ATSC Standard indicates that frequency drift can be addressed through the occasional insertion or deletion of a NULL packet from the transport stream. However, such NULL packet insertion/deletion reduces the transmission efficiency of the digital studio.
Therefore, a need exists in the art for a method and apparatus for splicing transport streams from multiple sources without violating the clock slew rate specified for a receiver.